Display Station - Terminal

Enhanced Definition

A display station, commonly referred to as a terminal in the mainframe context, is an input/output device used by end-users to interact with mainframe applications and the z/OS operating system. Historically, these were dedicated hardware devices like the IBM `3270 terminal`, but today they are predominantly software emulators running on personal computers. Their primary purpose is to provide a character-based interface for accessing mainframe services. In the mainframe context, a display station or terminal refers to a device or software application used by end-users to interact with the z/OS operating system and its applications. Historically, these were dedicated hardware devices like the **IBM 3270 Information Display System**, but today they are predominantly software emulators running on personal computers or other client devices. Their primary purpose is to provide a user interface for accessing mainframe resources, executing transactions, and managing system operations.

Key Characteristics

- Character-Based Interface: Displays text and accepts character input, typically in a fixed-size grid (e.g., 24 rows by 80 columns).
- Block Mode Transmission: Unlike character-at-a-time terminals, 3270 terminals operate in block mode, sending an entire screen or field of data to the host only after the user presses an Enter or function key.
- Dedicated Keyboard: Features specific keys for mainframe interaction, such as PA (Program Attention), PF (Program Function), Clear, Enter, and Erase EOF.
- Communication Protocol: Historically used SNA (Systems Network Architecture); modern emulators primarily use TN3270 over TCP/IP to connect to VTAM on z/OS.
- Screen Attributes: Supports various display attributes like color, highlighting, blinking, and protected fields to guide user input and highlight information.
- Emulation: Modern access is almost exclusively via TN3270 emulators (e.g., IBM Personal Communications, ZOC, x3270) that replicate the behavior of a physical 3270 terminal.

Use Cases

- TSO/ISPF Interaction: Users log into TSO (Time Sharing Option) to execute commands, manage datasets, submit JCL jobs, and edit programs using ISPF (Interactive System Productivity Facility).
- CICS Transaction Processing: End-users interact with online CICS (Customer Information Control System) applications to perform business transactions, such as order entry, customer service, or financial inquiries.
- IMS/DB2 Application Access: Accessing applications built on IMS (Information Management System) or DB2 databases for data entry, retrieval, and updates.
- System Monitoring and Operations: System programmers and operators use terminals to monitor system health, issue operator commands, and respond to system messages.
- Application Development and Testing: Developers use terminals to write, compile, link-edit, and test COBOL, PL/I, or Assembler programs within the TSO/ISPF environment.

Related Concepts

Terminals are the user's window into the mainframe ecosystem. They communicate with VTAM (Virtual Telecommunications Access Method), which manages the network connections and routes data to the appropriate host applications like TSO, CICS, or IMS. The data displayed and accepted by the terminal is often formatted by screen mapping facilities such as BMS (Basic Mapping Support) for CICS or GDDM (Graphical Data Display Manager) for other applications, ensuring a consistent user experience.

Best Practices:

Secure Emulator Configuration: Configure TN3270 emulators with TLS/SSL encryption for secure communication, especially when accessing sensitive data over public networks.
Strong Authentication: Enforce strong authentication mechanisms (e.g., RACF, ACF2, multi-factor authentication) for all terminal logins to prevent unauthorized access.
Session Management: Implement automatic logoff for idle sessions to mitigate security risks and free up system resources.
Accessibility Features: Utilize emulator features that enhance accessibility, such as customizable fonts, colors, and screen readers, for users with specific needs.
Performance Optimization: Ensure network latency is minimized for TN3270 traffic to provide a responsive user experience, as block mode can be sensitive to delays.